My theory has always been that two separate costs makes two definitive impressions, and it's reinforced throughout eBay (particularly by the DSRs, which ultimately ask buyers to separate the postage cost from total price, if it wasn't free, and evaluate/judge it). I think that's part of the reason why so many people take issue with postage that is more than what the service costs - total price is not irrelevant, but when it's comprised of two individual components with a specific name attached to them (purchase price + postage), it still (if even just on a subconscious level) creates what I call a value impression because people focus on two prices individually then add them to get the total price.
The purchase price of $39.95 at first glance appears like better value than a purchase price of $45.95, and some people will focus on that sort of thing.
For an interesting anecdote... Several months ago, when free post seemed to be one of the primary factors in Best Match rankings, I decided to try it on my jewellery and actually most of them had a cheaper overall price than when they had postage added. My sales virtually stopped, so, a couple of weeks later, I switched back to my old pricing structure, which of course increased the total price. Within 48 hours I sold more than I did in the whole time they were free post.
